So-called film-flow packings have been used in biological waste water purification. The waste water is sprayed through the top of a tower provided with a packing of as large specific surface area as possible. The waste water flows downwardly as a film over the large surface area of the packing, and the microorganisms of the bacteria culture (i.e. the so-called biological film) developed on the packing surface utilize the organic matter present in the waste water for their vital function. A purified waste water is discharged at the bottom of the tower.
The packing elements having a significant effect on waste water purification, because the purification is ensured by the biological film covering the surface of said elements.
Because of their many advantages, synthetic block-like (module-type) packings (generally made of vacuum-formed sheets stacked together) are widely used for biological treatment of sewage.
Such packings, generally comprising numerous open ducts and having a specific surface area 100-200 m.sup.2 /m.sup.3 specific surface are arranged in orderly manner in the supporting structure.
These block-like packings arranged in orderly manner facilitate the high-speed prepurification of waste water with a high content of organic matter, they do not become clogged, and are insensitive to impulsive loads, but are not suitable for the complete purification of waste water, because the residence time of the waste water is too short and their specific surface area is not sufficient for this purpose.
For this reason, a water purification step, e.g. purification with active sludge had to be used after the film-flow packing to obtain the qualitative parameters of the purified waste water permissible in fresh waters as regulated by various standards.
Thus it has become necessary to produce a synthetic packing which enables effective waste water purification, the cost of investment and energy demand of which much lower than those of the active sludge processes, so that the purified waste water complies with the qualitative parameters set out in the standards. The plastic packings made for this purpose must have at least 250 m.sup.2 /m.sup.3 specific surface, and should provide sufficiently long residence for the waste water in the film-flow packing as to successfully carry out the decomposition of the organic matter present in the waste water by the deposited micro-organisms.
A further requirement imposed on an packing is the adequate mechanical stability permitting the building of high packing units without intermediate support in such a way that the lowest layer of the packing is capable of carrying the collective weight of the synthetic elements above it, the biological film and downflowing waste water without damage and deformation. In addition, the packing should be light in weight and economical of material.
Another important aspect is resistance of the synthetic material to chemical and corrosive effects; in addition, the packing should be able to be processed in a relatively simple way.
Several synthetic packings are known. However these fulfil only in part above listed requirements.
The most widely used among them is the plastic sheet packing according to the Hungarian Pat. No. 171 178, consisting of synthetic elements provided with open-ended hollow cylindrical fins transverse to the main axis. This packing has a relatively large specific surface area and good mechanical stability, but several drawbacks as well.
The cylindrical elements have anisotropic mechanical characteristics in that the cylinders perpendicular to the axis are mechanically much weaker than in the axial direction, and this is counterbalanced only in part by the fins arranged crosswise. However, the supporting structure statically must be designed for the weak points of the packing unit. On the other hand, in terms of purification technology, the waste water flowing down on the interior of the horizontal cylinders of the packing is not capable of forming a film. Consequently these elements of the packing do not participate in the waste water purification, and are regarded as dead areas. In addition, even those cylinders which participate in purification are not used over their total internal surface area in the waste water purification, process, when their axes are at a certain minimal angle to the horizontal. In this case the water flowing down in the cylinders can accumulate at the bottom without wetting the upper part of the internal surface.
A further drawback of this packing is that although the fins formed on the cylinders for stability reasons theoretically extend the specific surface area of the packing, the biological film developed on the cylinders fills the spaces between the fins under operating conditions, breaking off with difficulty. As a result, in the dynamic balance developed during the continuous operation of the packing, the active specific surfaces readily participating in the waste water purification scarcely exceeds the value which can be achieved with plain, finless cylinders.
Another similar packing is described in the German Patent document DE-PS No. 2 558 986. This packing consists of cylinders with concentric walls, and the outer surfaces of the cylinders are interconnected with lamellae perpendicular to the cylinders.
These elements--due to their construction--have the same disadvantages as the one described in the Hungarian patent, i.e. they have anisotropic mechanical characteristics (they are less resistant to load perpendicular to the axis of the cylinder than to axial loads). Furthermore in terms of purification technology, the internal surface of the horizontal or nearly horizontal pieces of pipe is not wetted by the waste water. Consequently these surfaces cannot participate in the waste water purification.
In the U.K. Pat. No. 1 275 116 again a cylindrical element is described, the mantle of which is perforated, and reinforcing lamellae are provided in the cylinder.
The perforations enable the intensive mixing of the uncharged downflowing waste water, thereby resulting in wetting of the entire surface of the packing. The perforations result in a certain reduction of the specific surface as well.
A mechanical point of view, the packing has the earlier-mentioned diadvantage of the cylindrical form, which is increased by a further weakening by the perforations (also in the direction perpendicular to the axis).
In the German patent document DE-PS No. 2 928 784 a spherical element containing radially curving wall surfaces is described. Ducts are located in the centre of the spheres, allowing as much wetting of the packing as possible.
The radial arrangement of the wall surfaces does not, however allow their arrangement in sufficient density to increase the specific surface without the risk of clogging. The specific surface area for example of 32 m.sup.2 /m.sup.3 of a sphere with 7 cm diameter, is a fairly low value considering the parameters of other known, packings, since at least twice this value is required.